Extracorporeal membrane oxygenation (ECMO) remains the primary method of long term support after myocardial stunning caused by cardiac surgery in infants and children. ECMO often provides a bridge to recovery in these young patients. However, ventricular unloading as occurs with ECMO also promotes cardiac atrophy. Therefore, this therapy can be counterproductive in initiating reparative processes leading to restoration of normal cardiac function. Substantial abnormalities in hormonal homeostasis, such as decreases in circulating levels of thyroid hormones, occur during both shorter term cardiopulmonary bypass (CPB) and longer term ECMO. Disruptions in thyroid hormone homeostasis can alter substrate utilization, deplete citric acid cycle intermediates, possibly effecting net protein turnover. Additionally, we have noted that pyruvate supplementation can improve cardiac function after CPB in immature pigs. Thyroid hormone supplementation promotes pyruvate entry into the citric acid cycle, and promotes citric acid cycle intermediate conversion to amino acids. These findings suggest that appropriate substrate supplementation can improve protein synthesis and functional recovery after protracted mechanical circulatory support. We will study a prolonged period of mechanical circulatory support (ECMO) in the immature pig, an appropriate translational model for children undergoing these procedures. We will test the primary hypothesis: in the developing heart cardiac dysfunction due to ventricular unloading (ECMO)-is a consequence of impaired substrate utilization due at least in part to disruptions of thyroid hormone homeostasis. Targeted metabolic interventions in combination with thyroid hormone supplementation will minimize the adverse effects of ECMO and thereby improve longer term functional recovery and survival. Using NMR and GC-MS, we will determine if metabolic abnormalities, which lead to cardiac dysfunction and atrophy can be treated by supplementing the citric acid cycle with pyruvate. We will determine if pyruvate combined with thyroid hormone supplementation (T3) a) accelerates pyruvate flux, b) reduces oxidation of amino acids, c) stimulates transamination to amino acids and d) improves cardiac function and protein synthesis after a prolonged period of ventricular unloading. We will also determine if supplementation of medium chain fatty acids with and/or without thyroid hormone similarly supports the heart.